1. Field of the Art
The present invention relates generally to an optical reading apparatus for photoelectrically reading images on a recording medium, and more particularly to an optical reader of the type in which a beam of light emitted from a light emitter is transmitted through a light-emitting optical fiber and emitted therefrom to irradiate a reading spot on the surface of the recording medium, and the light beam reflected from the medium surface is transmitted through a light-receiving optical fiber to a light-sensitive element, an output of which is processed to provide an electric signal indicative of a reflectance of light in the reading spot on the medium.
2. Related Art Statement
One of the inventors named in the present application collaborated with another person to develop an optical reading apparatus of the above-indicated type, which is disclosed in U.S. patent application Ser. No. 726,314 which was filed Apr. 23, 1985 and was owned by the assignee of the present application at the time the present invention was made. In the disclosed optical reading apparatus, each reading unit is constituted by one light-emitting optical fiber and one light-receiving optical fiber. These two optical fibers are fixed on a reading head such that their ends positioned opposite to the surface of a recording medium are disposed adjacent to each other. For improved reading efficiency, the reading head has a plurality of such reading units. To assure an accurate reading of an image with a high resolution, the image-reading portions of those reading units opposite to the medium surface are spaced apart from each other at a pitch as small as possible.
In such an optical reader, it is required to read only a reading spot which is an overlapping area wherein an illumination spot irradiated by a light beam emitted from a light-emitting optical fiber overlaps a reflection spot within which a light beam reflected by the medium can be received by a light-receiving optical fiber. Namely, only the light beam reflected from the overlapping area (reading spot) should be received by the light-receiving optical fiber, in order to improve the reading resolution. In the optical reader as discussed above, however, it is inevitable that the reflection spot of a given light-receiving optical fiber overlaps the illumination spots of two or more light-emitting optical fibers. Therefore, if light beams are emitted from these plural light-emitting optical fibers at the same time, the light-receiving optical fiber in question receives light beams which is reflected from areas outside the corresponding reading spot. This leads to noises of an electric output of the reader, degrading its accuracy of reading. In view of this inconvenience, it is proposed to activate light-emitting elements corresponding to the light-emitting optical fibers, in a specially determined order as suggested in the above-identified U.S. Patent Application. However, this proposal requires a complicated control system, which inherently pushes up the cost of manufacture of the optical reading apparatus.
Further, a minimum pitch of the reading spots in the above apparatus is limited by diameters of a light-emitting optical fiber and the corresponding light-receiving optical fiber. To reduce the pitch, the reading head must use optical fibers having a reduced diameter. A reduction in diameter of the optical fibers used means an increased number of the optical fibers required, which requires an accordingly increased number of light-emitting elements and light-sensitive elements. This also results in an increase in the cost of manufacture of the reading apparatus. In addition, the optical fibers have their minimum diameter permissible for their intended function.
In the optical reading apparatus indicated above, the reading head is fed in an intermittent fashion. More specifically, the reading head reads all reading spots corresponding to the image-reading portions of all reading units on the reading head while the reading head is stopped at a given point on the medium. That is, the reading head is fed by a predetermined increment after the reading spots opposite to all image-reading portions have been read. In this connection, it will be understood that it is difficult to assure exact positioning of the reading head while permitting a high feed rate of the reading head. In this arrangement, therefore, it is impossible to increase the reading efficiency.
If it is desired to effect an image reading operation by using a reading head which covers a substantially entire width of a recording medium, the reading head must be equipped with a large number of reading units, that is, a large number of light-emitting elements, light-sensitive elements and control circuits therefor.
In the above case, the reading head requires a complicated structure for supporting the light-emitting and light-receiving optical fibers in a suitably fixed manner. Hence, the manufacturing cost is increased.